1. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus which prepares B-mode images and Doppler-mode images. The present invention also relates to a method of preparing B-mode images and Doppler-mode images.
2. Discussion of the Background
In an ultrasound diagnosis apparatus, it is known to often display both Doppler-mode images (or Doppler spectrums) and B-mode images (or tomograms) of a specimen such as, for example, a patient substantially in real time. To accomplish such a display, two techniques shown in FIGS. 1A and 1B are known as its examples. FIG. 1A is an illustration for explaining one of the techniques which may be called an interleaving a scan. FIG. 1B is an illustration for explaining the other one of the techniques which may be called a segment scan. In FIGS. 1A and 1B, ‘B’ represents a B-mode scan or a B-color-mode scan (hereinafter referred to as a B-mode scan), and ‘D’ represents a Doppler-mode scan.
As shown in FIG. 1A, one Doppler-mode scan is conducted every after three B-mode scans. Since the Doppler-mode scan is regularly repeated at a constant interval, its sampling frequency cannot be made higher than an actual rate frequency. In addition, artifacts may often appear in the Doppler-mode images and the B-mode images because of echo signals remaining mutually between in the Doppler-mode and the B-mode.
As shown in FIG. 1B, a non-Doppler segment period and a Doppler segment period are alternately repeated. In the non-Doppler segment period, the B-mode scans are conducted a plurality of times. In the Doppler segment period, the Doppler scans are conducted a plurality of times. Accordingly, it may be possible to avoid the problems occurring in the interleaving scan. However. Doppler signals resulting from the Doppler scans are not available for the non-Doppler segment periods, and accordingly, it is necessary to conduct interpolation processing of Doppler signals with respect to the non-Doppler segment periods. In the interpolation processing, a missing signal estimation (MSE) technique may often be used. According to the U.S. Pat. No. 4,559,953, for example, it may be at most about ten milliseconds (10 ms) when the living body is construed to be in a steady condition. Therefore, a period of about ten milliseconds may be a maximum predictable period in the interpolation processing.
In the ultrasound diagnosis apparatus, linear predictive coefficients are calculated based on steady signals in the body of the specimen so as to determine an auto-regressive model (AR model). Accordingly, linearly predicted signals as the AR model are generated with Gaussian noise as a resource of the linearly predicted signals in order to interpolate missing signals with respect to the non-Doppler segment period The difference between the predicted signals and unobtained actual Doppler signals is construed as an error. When the body of the specimen is in a steady condition, the error is small while the error becomes large when the body is in an unsteady condition. The large error leads to vertically-striped Doppler-mode (or spectrum) images resulting from an unsmooth connection between one segment and the next segment.
Such problematic images may noticeably be produced particularly in the small number of AR parameters, for example, when there is small number of samples (or scans) in the Doppler segment period and/or when a frame rate is low. This problem may be avoided by increasing the frame rate and/or the number of samples (or scans) in the Doppler segment period. This increase, however, causes another problem. It is not possible to decrease a range of Doppler velocity. In addition, since each interval between one non-Doppler segment period and the next, that is, between B-mode scans in one non-Doppler segment period and the next, becomes too long, a stripe due to a time phase difference between the one non-Doppler segment period and the next appears in a B-mode image, particularly when the B-mode image is a B-color-mode image. Accordingly, the B-mode image quality noticeably becomes deteriorated.
To solve this problem, it is known to decrease the prediction error of Doppler signals with specimen's body signals correlating to the Doppler signals as described in, for example, paragraphs [0019] to [0038] of Japanese Patent Application Publication No. 2001-149370.
As described above, an image quality of a B-mode image (or B-mode image quality), an image quality of a Doppler-mode image (or Doppler-mode image quality) and a frame rate may be determined by setting the Doppler segment period and the non-Doppler segment period. The B-mode image quality, the Doppler-mode image quality, and the frame rate are mutually correlated. Accordingly, the Doppler segment period and the non-Doppler segment period are preferably not determined uniquely or fixed, but may be expected to be freely changed according to a diagnosis part, an image display style, a user's preference, and/or the like.